1. Field of the Disclosure
The present disclosure relates generally to a fuser assembly for an electrophotographic imaging device and particularly to a fuser assembly having a heat transfer system which removes excess heat from a portion of the fuser assembly.
2. Description of the Related Art
In a belt fuser assembly for an electrophotographic imaging device, an endless belt surrounds a ceramic heating element. The belt is pushed against the heating element by a pressure roller to create the fusing nip. The heating element, typically a thick-film resistor on a ceramic slab, extends the full width of the printing process in order to suitably heat and fuse toner to the widest media sheets used with the imaging device. The fusing heat is controlled by measuring the temperature of the ceramic slab with a thermistor that is held in intimate contact with the ceramic and feeding the temperature information to a microprocessor-controlled power supply in the imaging device. The power supply applies power to the thick-film resistor when the temperature sensed by the thermistor drops below a first predetermined level, and interrupts power when the temperature exceeds a second predetermined level. In this way, the fuser assembly is maintained at temperature levels suitable for fusing toner to media sheets without overheating.
When printing on media sheets having widths that are less than the widest media width on which the imaging device can print, the media sheet removes heat from the fuser assembly in the portion of the fuser that contacts the media. Because the portion of the fuser assembly beyond the width of the media sheet does not lose any heat through the sheet, this second portion of the fuser assembly becomes hotter than the portion of the fuser assembly which contacts the media sheet. In order to prevent thermal damage to components of the fuser assembly, steps are taken to limit the overheating of the second portion of the fuser assembly. Typically, the inter-page gap between successive media sheets being printed is increased when media sheets less than the full width are used, thereby reducing the rate at which thermal energy is introduced through the fuser but at the expense of decreasing the process speed of the imaging device.
As imaging device speeds increase, the tolerable range of media width variation at full speed becomes smaller. In the case of imaging devices operating at 60 pages per minute (ppm) and above, a media width difference of 3 mm to 4 mm is seen to cause overheating in the small portion of the fuser assembly which does not contact the media sheet. For example, because letter paper and A4 paper differ in width by 6 mm, with A4 paper being narrower, an imaging device designed for printing on letter width media sheets and operating at 60 ppm or greater is seen to cause the portion of the fuser not contacting the media sheet to overheat if A4 paper is used, with the result that a letter width imaging device will necessarily slow down when printing on A4 media.
One approach to print on both letter and A4 width media at full process speeds using a letter width imaging device is to have two different fuser mechanisms—one fuser mechanism having a heater of the correct length for A4 media, and a second fuser mechanism having a heater for letter width media. However, problems occur if the fuser mechanism selected for a print job does not match the media sheet width. If the fuser mechanism associated with letter width printing is used for a print job using A4 media sheets, the fuser assembly may overheat as explained above. Conversely, if the fuser mechanism associated with A4 width printing is used for a print job using letter width media, the toner on the outermost 6 mm (for an edge-referenced imaging device) of the printed area is not sufficiently fused to the letter width media sheet.
Based upon the foregoing, a need exists for an improved fuser assembly for use with printing on narrower media sheets.